1. Field of the Invention
The present invention relates to an apparatus and method for measuring thermal noise. In particular, the present invention provides a vacuum-isolation vessel for use in measuring thermal noise in a microphone disposed within the vessel.
2. Discussion of the Related Art
The term "thermal noise" denotes the output signal of a microphone, in the absence of incident sound, due to stochastic processes such as the Brownian motion of air molecules impinging upon the microphone membrane or the Johnson noise generated in a microphone preamplifier. Measurement of the thermal noise is impeded by the difficulty of isolating the microphone from the ever present sounds and vibrations in the environment. The thermal noise is an important specification because it determines the minimum detectable sound pressure by the microphone.
Prior methods for determining the thermal noise in microphones include use of the conventional isolation vessel, the anechoic chamber, and the dummy microphone. The conventional isolation vessel contains massive walls to isolate an interior microphone from exterior environmental sound. The anechoic chamber, a room large enough to contain the entire noise measurement system, utilizes sound-absorbing wedges on its walls to ensure a quiet environment. The dummy microphone contains a fixed capacitor as a substitute for the microphone cartridge and is thus insensitive to incident sound.
The acoustical isolation provided by both the conventional isolation vessel and the anechoic chamber fails at low frequencies, typically below several hundred Hz for the former and below 40-100 Hz for the latter. The dummy microphone yields a false indication of thermal noise because it excludes the Brownian motion component and introduces additional components of noise not found in the microphone.
U.S. Pat. No. 3,104,543 to Kaminski discloses an acoustical vibration test device which utilizes an isolation chamber with inner and outer vessels. However, the inner vessel is not vibrationally isolated from the outer vessel causing the inner vessel to reverberate. This is suitable for testing the effect of noise on test objects but not for measuring thermal noise.
Another test apparatus is disclosed by U.S. Pat. No. 3,827,288 to Fletcher et al. which also utilizes a reverberant acoustic chamber that does not isolate a test object vibrationally.
U.S. Pat. No. 3,698,241 to Bouclin discloses a method for vibrational testing in which a test object is suspended within a vessel. However, the vessel is not acoustically isolated.
Conventional isolation apparatus and methods have been unable to provide an acoustically quiet and vibration-free environment for accurately measuring thermal noise.